A spline is a series of parallel, evenly spaced axially running grooves formed around the circumference of a cylindrical workpiece. A spline may, for example, be formed at the end of a shaft which is to be attached to a wheel or gear to transmit torque. The wheel or gear has an inner diameter formed with axially running teeth to mate with the spline teeth when the shaft is inserted into the inner diameter. The resulting connection between the two components permits the transmission of rotary force with zero possibility of slippage.
Splines are most commonly formed by a cold-working process wherein the cylindrical workpiece is rolled between a pair of parallel dies, called racks, which have teeth of the proper size and spacing to create the desired grooves in the workpiece. The workpiece is placed between the tooth-bearing surfaces of the two racks, pressure is applied perpendicular to the plane of the racks and they are then moved past each other in a stroke which causes the workpiece to make several revolutions between the toothed surfaces, the teeth plastically deforming the material of the workpiece to form grooves. On some racks, the teeth which contact the workpiece at the beginning of the stroke are very shallow in depth, successive teeth increasing in depth gradually along the length of the rack until reaching the full, desired depth. By "ramping up" the tooth depth in this fashion, material displacement is progressive and less instantaneous pressure is required. This splining process is described in detail in U.S. Pat. No. 3,982,415, the disclosure of which is incorporated herein by reference.
At the beginning of the spline forming stroke, the surface of the workpiece is smooth so that it is only the surface friction between the workpiece and the tops or lands of the teeth that causes the workpiece to roll as the racks are moved with respect to one another. To achieve high precision in the geometry of the spline being formed it is necessary that the workpiece roll between the racks in a precise and repeatable manner, i.e., there must be no slippage between the workpiece and the racks. A high coefficient of friction between the workpiece and the land surfaces of the teeth is therefore desired.
The teeth are formed on the racks by a grinding operation that leaves the teeth with a very smooth surface. According to the prior art, a sandblasting procedure is used to roughen the surfaces of the lands thereby to increase the coefficient of friction between the workpiece and the lands. Such sandblasting has two undesirable effects. First, the sandblasting cannot be practically confined only to the lands of the teeth, but rather abrades all of the tooth surfaces with a resulting degradation in the dimensional accuracy of the rack and hence the spline it is used to form. Second, sandblasting pits the surfaces of the teeth thereby weakening the metal and leading to more rapid wear and dulling of the racks during use, and consequently necessitating more frequent rack replacement.